This description relates to generally a hybrid electric vehicle (HEV), and more particularly relates method for controlling HEV power-train.
Hybrid electric vehicle and its power-train configuration are well known and presented in such as U.S. Pat. No. 6,702,053 and a Japanese magazine article “Shinichi ABE, HEV Control—Performance improvement of New Prius, ENGINE TECHNOLOGY, June 2004 issue”, where an internal combustion engine, a first electric machine and vehicle driving wheels are rotationally coupled through a power transmission system, such as a planetary gear set, having three rotational elements such as a planetary carrier connected to the internal combustion engine, a sun gear connected to the first electric machine and a ring gear connected to the driving wheels. In addition to that, a second electric machine is rotationally coupled to the driving wheels with a fixed speed reduction ratio.
The operation of this type of HEV power-train can be illustrated by the relationship of rotational velocities between its three rotational elements. The relationship can be expressed as a collinear diagram as shown in FIG. 4, where the velocities of the three elements always sit on an operational collinear line LC which changes its position and orientation based on torque balance acting on the three elements. Engine torque TENG is transferred through the planetary carrier to the sun gear and the ring gear in a fixed torque split ratio (TES/TER=constant) defined by number of teeth of the planetary gear set. FIG. 4 shows a so-called positive torque split mode, where the engine is controlled to generate desired output power at a desired engine speed at an operating point where less fuel is consumed (i.e. an efficient operating point). The desired engine speed can be achieved, based on the collinear relationship in FIG. 4, by controlling the first electric machine to operate as an electric generator. This allows the first electric machine to brake the sun gear by resisting the torque TES transferred from the engine to the sun gear and generate electricity. Note that the ring gear velocity directly corresponds to the driving wheel velocity, so it can be considered fixed. In general, the electricity generated by the first electric machine is used for driving the second electric machine that is operated as a motor to supplement the torque TER transferred from the engine to the ring gear. In other words, all of the power generated by the engine may be transmitted to the driving wheels.
On the other hand, the efficiency of the positive power-split mode may be reduced, when the desired power at the driving wheels is low particularly at a low vehicle speed. During these conditions, the desired engine power may not be coincident with efficient engine operating conditions. For example, an engine can have a lower speed constraint to ensure that engine vibration and combustion stability are acceptable. The engine operates efficiently at this speed when the engine torque is at a particular level. However, if the desired engine torque is less than this level, the engine torque can be reduced to match the desired level, but at the expense of engine efficiency. The '053 patent describes a method to improve engine efficiency while operating in the positive power-split mode by changing the air-fuel ratio, thereby moving the efficient engine operating point toward a lower torque side while keeping the lower engine speed constraint. However, this method does not recognize another problem occurring during a high speed cruising where demanded torque is low but speed is high, as described below.
In addition to the positive power-split mode, a so-called a negative power-split mode may occur during the high speed cruising. At this condition, engine speed is lowered to meet the low power requirement of the driving wheels. The engine speed is reduced, as shown by a dotted collinear line LC of FIG. 5, by rotating the first electric machine in an opposite direction to that in the positive mode (i.e. a negative velocity). The first electric machine achieves a negative velocity by rotating against the torque TES transferred from the engine. Therefore, the first electric machine needs to operate as a motor and to consume power that is typically supplied from the second electric machine. Consequently, the second electric machine is operated as an electric generator in this mode. Further, the torque TES, a part of the torque generated by the engine and transferred to the sun gear, is never transmitted to the driving wheels but is dissipated by the first electric machine that is driving the sun gear and using energy. Also, a part of the torque TER transferred from the engine to the ring gear is used by the second electric machine generating the energy (electricity) to drive the first electric machine (energy re-circulation), which leads to reduced power transmission efficiency.
The ENGINE TECHNOLOGY article describes a method to improve the reduced efficiency in the negative power-split mode by selecting an engine operating point (e.g., engine torque and speed) that takes into account both the engine operating efficiency and the power transmission efficiency. Specifically, the method moves the engine operating point toward a higher speed and a lower torque, so as to reduce the amount of energy circulating by decreasing speed of the first electric machine.
Although this method attempts to lower the amount of energy circulating through the system and to improve the power-train efficiency by reducing the speed of the first electric machine, it also can lower the engine operating efficiency because the engine operates at a condition where engine pumping losses may be increased. In other words, the method exchanges the engine efficiency for the power transmission efficiency and therefore leaves room additional system efficiency improvement.
The inventors herein have recognized the above described disadvantages of the prior arts and need to improve of the system efficiency of the hybrid electric vehicle power-train over the above described prior arts.